Federation can get messy when you start to compare instances between each other.
- For one some instances disable down votes, so may count differently.
- Then you might see delays in synchronisation for whatever reason.
- Finally, federation & defederation. suppose you have instance A, B, C, and D but instance A has blocked C. Users on C comment in and upvote content on D. Instances A&B will download a copy of the stuff posted by D users, but only B will have upvotes contributed by C. Therefore, different upvotes counts.
And that's just what I am aware of.